^^^In American english a bi-turbo would be sequential instead of parallel or "twin" turbos.....kinda like Siamese twins versus actual separate identical twins (in that Siamese twins share some "piping"...LOL)

and the difference between a bi-turbo and a twin turbo is what exactly?

A biturbo arraignment has two different sized turbos (usually a small one for low engine speed and a large one for the upper range) that share the same exhaust piping so they are known as sequential, because the large one is downstream of the small one (several BMW diesels are bi turbos)

This is similar to the idea of a twin scroll turbo like on the N55, but bot quite as both the small and large turbo impellers share the same housing in a twinscroll. Think of a twinscroll as a tightly packaged Bi Turbo

A twin turbo has two identical turbos that get fed off of two different headers. Twin turbos on a 6 are two banks of 3 cylinders, twins on an 8 pot are two banks of 4 cylinders, etc...

And you can't simply swap turbos on the N54. The turbo housings are cast into the header. So you Either need a custom header made or the second, easier option, which is to put a more efficient turbo possible into the stock housing (this is what ASR and RB do)